Section 42(4)(c) in The Central Goods and Services Tax Rules, 2017
(c)where, C3aggregate_comm exceeds C3final_comm, such excess shall be reversed by the registered person in Form GSTR-3B or through Form GST DRC-03 in the month not later than the month of September following the end of the financial year in which the completion certificate is issued or first occupation takes place of the project and the said person shall be liable to pay interest on the said excess amount at the rate specified in sub-section (1) of section 50 for the period starting from the first day of April of the succeeding financial year till the date of payment;